This past Friday and Saturday ,(April 3rd and 4th), marked 46 years since the second largest single-day tornado outbreak, topped only by the April 2011 outbreak. 13 states, from the Great Lakes to the Deep South were affected. 148 tornadoes were recorded. Of those 148, 95 were F2 or stronger and 30 were F4 or F5. 335 people were killed and another 6000 were injured.
A powerful low pressure system developed over the Interior Plains on April 1, 1974. A surge of extremely moist air intensified the system as it approached the Mississippi and Ohio Valleys. A severe weather outbreak was expected. Watches were issued the morning of the outbreak. All the ingredients began to come together that morning as well. The low deepened, moisture and instability increased rapidly, and large scale lifting overspread the warm sector. A large complex of storms spread into the Ohio and Tennessee Valleys and evolved into three main lines of tornado-producing storms. The first line of storms moved quickly northeast.
The Second Line of storms:
By early afternoon on April 3rd, the complex of storms split; the southern portion lagged and the northern portion sped up. Several supercells began to form in the warmer, moister, southern portion. Meanwhile, a new area of storms had begun to form over Eastern Arkansas and Missouri. These increased in coverage and intensity as the approached Illinois, Indiana, and Northern Kentucky. This line produced the first F5 tornado of the outbreak, near Depauw, IN. Two other F5 tornadoes were produced by this line: one in Xenia, OH, and the other in Brandenburg, KY.
The Third Line of Storms:
While the second line of storms was producing tornadoes in the warm sector, a third line of storm activity developed from near St. Louis into West-Central Illinois. By about 4:00 P.M on April 3rd, F3 tornadoes hit near both Decatur and Normal, Illinois. As storms moved into the warmer, moister air mass over eastern Illinois and Indiana, longer-lived tornadoes were produced. Later, wind shear increased in the south and produced numerous supercells, which produced strong, long-tracked tornadoes from southern Tennessee into Eastern Mississippi. These storms produced some of the strongest of the outbreak.
In total, 21 tornadoes hit 38 different counties in Indiana. 47 people were killed and 800 others were injured. According to the National Weather Service, most of the tornadoes moved at nearly a mile a minute. 3 F5 tornadoes touched down in Indiana. That was the only time since 1950 that an F5 tornado touched down in the state. In addition, 6 F4 tornadoes also hit Indiana, including one just about 15 minutes east of Muncie, in Parker City, and 2 in near Madison in Southern Indiana. Several other F1, F2, and F3 tornadoes also touched down across the state.
In the aftermath, many lessons were learned, and changes were made. Prior to the outbreak, not every National Weather Service office had a radar, and detecting tornadoes was extremely difficult due to absence of velocity data. Meteorologists relied heavily on the presence of hook echoes and spotter reports for warning issuance. Since the outbreak, “improvements in communications, warning systems, emergency preparedness, and forecast techniques,” have led to “increased lead times on warnings, more accurate forecasts, greater public awareness and more reliable communications.”
For more information (and a couple videos) :
https://www.weather.gov/ind/april3_1974tor https://www.weather.gov/media/lmk/pdf/apr3_1974/corfidi_paper.pdf http://www.ustornadoes.com/2013/04/03/looking-back-at-the-april-3-4-1974-super-outbreak/ https://en.wikipedia.org/wiki/1974_Super_Outbreak https://www.youtube.com/watch?v=bVHcvaWAoks https://www.youtube.com/watch?v=nW8_bLG3VdI